I was wondering about the depth of the vagina. I've read statistics that say that the average vagina is only 3 to 4 inches deep. This seems way too small to me, since the average penis is considerably longer than that. Wouldn't that mean that most penises would crash into the cervix repeatedly during intercourse? Since this obviously doesn't happen, my question is this: does the vagina actually elongate during intercourse to accommodate the entire length of the average penis?
Yes, just as the vagina has the capacity to expand, allowing for the passage of a baby during childbirth, the vagina also has the ability to elongate during intercourse to accommodate a penis. As you mentioned, for some women, the depth from the vaginal opening to the tip of the cervix is 3 to 4 inches when they are not sexually aroused. Other women may have a vaginal depth of five to seven inches. Regardless, during arousal, blood flows to the genital area, and sexual excitement causes the upper two-thirds of the vagina to lengthen by forcing the cervix and uterus to ascend. The vagina also lubricates to help ease penetration.
Some people think that the vaginal canal is a continuously open space. However, this is a misperception. Think of the vaginal canal as if it were a balloon that is not filled with any air. The walls, which have the potential to expand and elongate, gently touch one another. When something is placed inside, they mold around the width and accommodate the length of a penis, tampon, finger(s), or sex toy.
Sometimes during penetration, a penis or other object inserted in a vagina does hit the cervix. This may be an indication that the woman is not physiologically aroused enough; when she is more aroused, her vagina will elongate and her cervix, the neck of the uterus, will lift up and move out of the way. Other times, contact with the cervix can happen if a penis is larger than average or if the thrusting is too deep. Communicating with a partner about the discomfort — "Ouch, that's a little too deep" — and changing sexual positions may be helpful.